fordfan sounds right. Is your check engine light on? How many miles does your engine have on it? Are you noticing your oil level going down?
If the CEL is on, get the code read. It may be as simple as a misfire. And you also may be able to spend the money to just swap out the spark plugs.
If you keep up with the maintenance schedule and you've got less than 12,000 miles per year, or less than 100-150k total I'd be skeptical that you were losing compression. You could always check the compression on that cylinder. If the compression is good, then it's probably just a fouled plug.
If you're not noticing any oil burning then most likely you won't have enough seeping up through the rings to foul the plug. You may want to check your PCV valve. I have seen some cases where a stuck pcv valve lets pressure build up inside and force oil down through the valve guides, or up through the rings.
One last if... If the deposits on the plug are matte black not shiny black, then you have carbon fouling. That would tend to indicate that you aren't getting good ignition in that cylinder and lead me to guess that your COP or coil pack is starting to go, if it hasn't gone already.
[edit: Once you have the plug out you can compare it to the chart at http://www.verrill.com/moto/sellingg...colorchart.htm
And last but not least... Good luck, let us know how it goes.